The U.S. Standard Aerospace 1159 relates to a configuration which may be used for the external drive formation of flange screws and flange nuts, as well as for the internal drive formation of screws having a cylindrical head or having a head of the recessed type.
Generally, the said Aerospace standard 1159 configuration may be described as a configuration which, when viewed in plan, comprises a cylinder having a plurality of grooves defined in part by re-entrant part-cylindrical surfaces and equiangularly spaced about its periphery, the regions of intersection of the grooves with the cylinder being curved and the radius of the part-cylindrical groove being many times larger than that of said curved region of intersection.
In my copending allowed patent application, U.S. Ser. No. 445,920 for a CONFIGURATION FOR NUTS AND BOLT HEADS, filed in the U.S. patent office on Feb. 26, 1974, now U.S. Pat. No. 3,969,974, I have disclosed that, as far as external drive formations are concerned, the curved regions of intersection of the part-cylindrical grooves with the cylinder may advantageously be substituted by planar surfaces.
As set forth in my said co-pending patent application Ser. No. 445,920, such planar surfaces at the regions of intersection may also be used with external drive formations of a slightly different type. These slightly different external drive formations similarly, when viewed in plan, comprise an outer cylinder wherein are sunk a plurality of grooves. The grooves have a curved bottom surface which merges with two planar inclined side surfaces. Nuts and bolts having an external drive formation of this last mentioned type are presently brought on the market under the registered Trade Mark HIGRIP.
Internal drive formations according to the present U.S. Standard Aerospace 1159 suffer from two drawbacks, one being that the screw driving tools to be used for tightening screws having such an internal drive formation are unnecessarily expensive in manufacture, and the other one being that this particular internal drive formation does not permit use of screw driving tools of different designs, such as those adpated primarily to be used with screws of the so called TRI-SQUARE type and similar tools.